Both EU and Northern Ireland domestic law requires the government to do an emissions test on diesels at MOT using a machine called a smoke meter.

The tests were introduced at MOT in Northern Ireland in 2006 but were suspended 12 weeks later due to concerns about health and safety in the testing halls.

They have never been reintroduced.

The smoke meter test is routinely carried out in the rest of the UK where MOTs are done in private garages rather than state-run centres.

The Driver & Vehicle Agency (DVA) said its staff do a "visual test" and make a "personal judgement" about emissions.

It means there are potentially tens of thousands of diesel cars with illegal emissions on the road in Northern Ireland.

According to Professor Robert Lee of Birmingham University, failure to do the test means the government in Northern Ireland is breaking the law.

He said it may now face legal action in Belfast and Brussels.

SDLP MLA John Dallat said the failure to do the tests was a "scandal" which required a "thorough inquiry". He added: "I'm sure there are very few people watching this programme that haven't seen cars emitting huge volumes of black smoke and wondering how on earth those things are on the road.

"Now we know. If those tests were there, those cars would be gone."

How come, with all that black smoke, nobody "noticed" that the law was being broken by the government for 12 years .....

The whole thing is a joke and a con. My old diesel Peugeot once failed it's emission test despite some tweaking and driving the car round the block at high revs. I took it to another garage and it passed.